1. Field of the Invention
The invention relates generally to systems and methods for signal modulation, and more particularly to systems and methods for pulse width modulating waveforms to represent asymmetric signal levels using pulses that are symmetric within their respective switching periods.
2. Related Art
In a typical pulse width modulation (PWM) amplifier, a digital audio signal (typically encoded in a pulse code modulation, or PCM, format) is processed and modulated to produce a PWM signal which is then used to drive an output stage which produces an analog output signal. Part of the processing that is typically performed on the PCM signal is a quantization of the signal to reduce the number of possible signal levels to a number which can be represented in the PWM modulation scheme used in the amplifier. For example, a 24-bit signal may be quantized to a 10-bit signal.
The number of signal levels that can be represented in a PWM signal is determined by the number of discrete steps, or levels, in a single PWM switch period. In each PWM switch period, the PWM signal will be high for some number of steps, and low for the remainder of the steps. The width of the pulse is the number of steps during which the signal is high. It is desirable for purposes of processing and correcting the signals in the amplifier to have the pulse (the steps during which the signal is high) centered and symmetric within the switch period. This also causes some difficulties, however, as requiring that the pulses be symmetric effectively eliminates half of the possible pulse widths, since an odd number of steps cannot be symmetrically centered within the PWM switch period (assuming an even number of steps in the period.) If, on the other hand, it is not required that the pulses be centered, there are a greater number of potential pulse widths, but it is difficult to correct for artifacts resulting from the use of asymmetric pulses.
It would therefore be desirable to have a way to represent a greater number of signal levels using the available steps in the PWM switch period, while at the same time maintaining the symmetry of the pulses within the PWM switch periods so the processing of the signals in the amplifier can be simplified.